In recent years, an amount of data handled by a computer has grown enormously due to rapid spread of Internet environments including the broadband. In order to efficiently process a large amount of data, data centers are rapidly increasing where numerous computers (servers) are installed in one room for the purpose of collective management.
Numerous servers are in operation in a data center and each of the servers generates heat corresponding to its power consumption. In such a data center, in order to avoid erroneous operations or failures of the servers attributed to the heat generated by the servers, air conditioners are installed in a server room to introduce low-temperature air to the servers, thereby preventing overheating of the servers.    Patent Document 1: Japanese Laid-open Patent Publication No. 2004-125410    Patent Document 2: Japanese Laid-open Patent Publication No. 2005-43231    Patent Document 3: Japanese Laid-open Patent Publication No. 2006-46922    Patent Document 4: Japanese Laid-open Patent Publication No. 2006-84176
In the meantime, energy conservation is preferred in every aspect of society in the light of prevention of global warming. Since a data center consumes enormous electric power for operating an air-conditioning system including air conditioners, efficient operation of the air-conditioning system is preferred.
In order for the efficient operation of the air-conditioning system, it is preferable to measure power consumption of servers and to feed such information back to control of the air-conditioning system. A current sensor using a shunt resistor or an AC current sensor using a current transformer has long been employed for measuring power consumption of an electric instrument.
However, implementation of measurement of power consumption of numerous electric instruments (the servers and the like) with the current sensors uses a large number of the current sensors as well as a large number of wires for connecting the current sensors to a measurement apparatus. As a consequence, not only costs for the current sensors and the measurement apparatus but also costs for arrangement of the current sensors (inclusive of installation of the wires) and costs for maintenance also become large. Accordingly, it is impractical to fit the above-mentioned current sensors respectively on the numerous electric instruments and to individually measure the power consumption of the electric instruments.